The sections below describe these tasks. When you are done with the
integration, you should be able to compile your application successfully and you
can begin testing, as described in Setting Up the Test
Environment.

Adding the Licensing Permission

To use the Google Play application for sending a license check to the
server, your application must request the proper permission,
com.android.vending.CHECK_LICENSE. If your application does
not declare the licensing permission but attempts to initiate a license check,
the LVL throws a security exception.

To request the licensing permission in your application, declare a <uses-permission>
element as a child of <manifest>, as follows:

<uses-permission
android:name="com.android.vending.CHECK_LICENSE" />

For example, here's how the LVL sample application declares the permission:

Note: Currently, you cannot declare the
CHECK_LICENSE permission in the LVL library project's manifest,
because the SDK Tools will not merge it into the manifests of dependent
applications. Instead, you must declare the permission in each dependent
application's manifest.

Implementing a Policy

Google Play licensing service does not itself determine whether a
given user with a given license should be granted access to your application.
Rather, that responsibility is left to a Policy implementation that you provide
in your application.

Policy is an interface declared by the LVL that is designed to hold your
application's logic for allowing or disallowing user access, based on the result
of a license check. To use the LVL, your application must provide an
implementation of Policy.

The Policy interface declares two methods, allowAccess() and
processServerResponse(), which are called by a LicenseChecker
instance when processing a response from the license server. It also declares an
enum called LicenseResponse, which specifies the license response
value passed in calls to processServerResponse().

processServerResponse() lets you preprocess the raw response
data received from the licensing server, prior to determining whether to grant
access.

A typical implementation would extract some or all fields from the license
response and store the data locally to a persistent store, such as through
SharedPreferences storage, to ensure that the data is
accessible across application invocations and device power cycles. For example,
a Policy would maintain the timestamp of the last successful license check, the
retry count, the license validity period, and similar information in a
persistent store, rather than resetting the values each time the application is
launched.

allowAccess() determines whether to grant the user access to
your application, based on any available license response data (from the
licensing server or from cache) or other application-specific information. For
example, your implementation of allowAccess() could take into
account additional criteria, such as usage or other data retrieved from a
backend server. In all cases, an implementation of allowAccess()
should only return true if the user is licensed to use the
application, as determined by the licensing server, or if there is a transient
network or system problem that prevents the license check from completing. In
such cases, your implementation can maintain a count of retry responses and
provisionally allow access until the next license check is complete.

To simplify the process of adding licensing to your application and to
provide an illustration of how a Policy should be designed, the LVL includes
two full Policy implementations that you can use without modification or
adapt to your needs:

ServerManagedPolicy, a flexible Policy
that uses server-provided settings and cached responses to manage access across
varied network conditions, and

StrictPolicy, which does not cache any response
data and allows access only if the server returns a licensed
response.

For most applications, the use of ServerManagedPolicy is highly
recommended. ServerManagedPolicy is the LVL default and is integrated with
the LVL sample application.

Guidelines for custom policies

In your licensing implementation, you can use one of the complete policies
provided in the LVL (ServerManagedPolicy or StrictPolicy) or you can create a
custom policy. For any type of custom policy, there are several important design
points to understand and account for in your implementation.

The licensing server applies general request limits to guard against overuse
of resources that could result in denial of service. When an application exceeds
the request limit, the licensing server returns a 503 response, which gets
passed through to your application as a general server error. This means that no
license response will be available to the user until the limit is reset, which
can affect the user for an indefinite period.

Returns the cached response for all license checks, for as long as the
cached response is valid, rather than making a request to the licensing server.
Setting the response validity according to the server-provided VT
extra is highly recommended. See Server Response Extras
for more information.

Uses an exponential backoff period, if retrying any requests the result in
errors. Note that the Google Play client automatically retries failed
requests, so in most cases there is no need for your Policy to retry them.

Provides for a "grace period" that allows the user to access your
application for a limited time or number of uses, while a license check is being
retried. The grace period benefits the user by allowing access until the next
license check can be completed successfully and it benefits you by placing a
hard limit on access to your application when there is no valid license response
available.

Designing your Policy according to the guidelines listed above is critical,
because it ensures the best possible experience for users while giving you
effective control over your application even in error conditions.

Note that any Policy can use settings provided by the licensing server to
help manage validity and caching, retry grace period, and more. Extracting the
server-provided settings is straightforward and making use of them is highly
recommended. See the ServerManagedPolicy implementation for an example of how to
extract and use the extras. For a list of server settings and information about
how to use them, see Server Response
Extras.

ServerManagedPolicy

The LVL includes a full and recommended implementation of the Policy
interface called ServerManagedPolicy. The implementation is integrated with the
LVL classes and serves as the default Policy in the library.

ServerManagedPolicy provides all of the handling for license and retry
responses. It caches all of the response data locally in a
SharedPreferences file, obfuscating it with the
application's Obfuscator implementation. This ensures that the license response
data is secure and persists across device power cycles. ServerManagedPolicy
provides concrete implementations of the interface methods
processServerResponse() and allowAccess() and also
includes a set of supporting methods and types for managing license
responses.

Importantly, a key feature of ServerManagedPolicy is its use of
server-provided settings as the basis for managing licensing across an
application's refund period and through varying network and error conditions.
When an application contacts the Google Play server for a license check, the
server appends several settings as key-value pairs in the extras field of certain
license response types. For example, the server provides recommended values for the
application's license validity period, retry grace period, and maximum allowable
retry count, among others. ServerManagedPolicy extracts the values from the
license response in its processServerResponse() method and checks
them in its allowAccess() method. For a list of the server-provided
settings used by ServerManagedPolicy, see Server Response
Extras.

For convenience, best performance, and the benefit of using license settings
from the Google Play server, using ServerManagedPolicy as your
licensing Policy is strongly recommended.

If you are concerned about the security of license response data that is
stored locally in SharedPreferences, you can use a stronger obfuscation
algorithm or design a stricter Policy that does not store license data. The LVL
includes an example of such a Policy — see StrictPolicy for more information.

StrictPolicy

The LVL includes an alternative full implementation of the Policy interface
called StrictPolicy. The StrictPolicy implementation provides a more restrictive
Policy than ServerManagedPolicy, in that it does not allow the user to access
the application unless a license response is received from the server at the
time of access that indicates that the user is licensed.

The principal feature of StrictPolicy is that it does not store any
license response data locally, in a persistent store. Because no data is stored,
retry requests are not tracked and cached responses can not be used to fulfill
license checks. The Policy allows access only if:

The license response is received from the licensing server, and

The license response indicates that the user is licensed to access the
application.

Using StrictPolicy is appropriate if your primary concern is to ensure that,
in all possible cases, no user will be allowed to access the application unless
the user is confirmed to be licensed at the time of use. Additionally, the
Policy offers slightly more security than ServerManagedPolicy — since
there is no data cached locally, there is no way a malicious user could tamper
with the cached data and obtain access to the application.

At the same time, this Policy presents a challenge for normal users, since it
means that they won't be able to access the application when there is no network
(cell or Wi-Fi) connection available. Another side-effect is that your
application will send more license check requests to the server, since using a
cached response is not possible.

Overall, this policy represents a tradeoff of some degree of user convenience
for absolute security and control over access. Consider the tradeoff carefully
before using this Policy.

A typical Policy implementation needs to save the license response data for
an application to a persistent store, so that it is accessible across
application invocations and device power cycles. For example, a Policy would
maintain the timestamp of the last successful license check, the retry count,
the license validity period, and similar information in a persistent store,
rather than resetting the values each time the application is launched. The
default Policy included in the LVL, ServerManagedPolicy, stores license response
data in a SharedPreferences instance, to ensure that the
data is persistent.

Because the Policy will use stored license response data to determine whether
to allow or disallow access to the application, it must ensure that any
stored data is secure and cannot be reused or manipulated by a root user on a
device. Specifically, the Policy must always obfuscate the data before storing
it, using a key that is unique for the application and device. Obfuscating using
a key that is both application-specific and device-specific is critical, because
it prevents the obfuscated data from being shared among applications and
devices.

The LVL assists the application with storing its license response data in a
secure, persistent manner. First, it provides an Obfuscator
interface that lets your application supply the obfuscation algorithm of its
choice for stored data. Building on that, the LVL provides the helper class
PreferenceObfuscator, which handles most of the work of calling the
application's Obfuscator class and reading and writing the obfuscated data in a
SharedPreferences instance.

The LVL provides a full Obfuscator implementation called
AESObfuscator that uses AES encryption to obfuscate data. You can
use AESObfuscator in your application without modification or you
can adapt it to your needs. If you are using a Policy (such as
ServerManagedPolicy) that caches license response data, using AESObfuscator as
basis for your Obfuscator implementation is highly recommended.
For more information, see the next section.

AESObfuscator

The LVL includes a full and recommended implementation of the Obfuscator
interface called AESObfuscator. The implementation is integrated with the
LVL sample application and serves as the default Obfuscator in the library.

AESObfuscator provides secure obfuscation of data by using AES to
encrypt and decrypt the data as it is written to or read from storage.
The Obfuscator seeds the encryption using three data fields provided
by the application:

A salt — an array of random bytes to use for each (un)obfuscation.

An application identifier string, typically the package name of the application.

A device identifier string, derived from as many device-specific sources
as possible, so as to make it as unique.

To use AESObfuscator, first import it to your Activity. Declare a private
static final array to hold the salt bytes and initialize it to 20 randomly
generated bytes.

Next, declare a variable to hold a device identifier and generate a value for
it in any way needed. For example, the sample application included in the LVL
queries the system settings for the
android.Settings.Secure.ANDROID_ID, which is unique to each device.

Note that, depending on the APIs you use, your application might need to
request additional permissions in order to acquire device-specific information.
For example, to query the TelephonyManager to obtain
the device IMEI or related data, the application will also need to request the
android.permission.READ_PHONE_STATE permission in its manifest.

Before requesting new permissions for the sole purpose of acquiring
device-specific information for use in your Obfuscator, consider
how doing so might affect your application or its filtering on Google Play
(since some permissions can cause the SDK build tools to add
the associated <uses-feature>).

Finally, construct an instance of AESObfuscator, passing the salt,
application identifier, and device identifier. You can construct the instance
directly, while constructing your Policy and LicenseChecker. For example:

For a complete example, see MainActivity in the LVL sample application.

Checking the License from an Activity

Once you've implemented a Policy for managing access to your application, the
next step is to add a license check to your application, which initiates a query
to the licensing server if needed and manages access to the application based on
the license response. All of the work of adding the license check and handling
the response takes place in your main Activity source file.

Overview of license check and response

In most cases, you should add the license check to your application's main
Activity, in the onCreate() method. This
ensures that when the user launches your application directly, the license check
will be invoked immediately. In some cases, you can add license checks in other
locations as well. For example, if your application includes multiple Activity
components that other applications can start by Intent,
you could add license checks in those Activities.

A license check consists of two main actions:

A call to a method to initiate the license check — in the LVL, this is
a call to the checkAccess() method of a LicenseChecker object that
you construct.

A callback that returns the result of the license check. In the LVL, this is
a LicenseCheckerCallback interface that you implement. The
interface declares two methods, allow() and
dontAllow(), which are invoked by the library based on to the
result of the license check. You implement these two methods with whatever logic
you need, to allow or disallow the user access to your application. Note that
these methods do not determine whether to allow access — that
determination is the responsibility of your Policy implementation. Rather, these
methods simply provide the application behaviors for how to allow and
disallow access (and handle application errors).

The allow() and dontAllow() methods do provide a "reason"
for their response, which can be one of the Policy values, LICENSED,
NOT_LICENSED, or RETRY. In particular, you should handle the case in which
the method receives the RETRY response for dontAllow() and provide the user with an
"Retry" button, which might have happened because the service was unavailable during the
request.

Code in the application's main Activity instantiates LicenseCheckerCallback
and LicenseChecker objects. When constructing LicenseChecker, the code passes in
Context, a Policy implementation to use, and the
publisher account's public key for licensing as parameters.

The code then calls the checkAccess() method on the
LicenseChecker object. The method implementation calls the Policy to determine
whether there is a valid license response cached locally, in
SharedPreferences.

If so, the checkAccess() implementation calls
allow().

Otherwise, the LicenseChecker initiates a license check request that is sent
to the licensing server.

Note: The licensing server always returns
LICENSED when you perform a license check of a draft application.

When a response is received, LicenseChecker creates a LicenseValidator that
verifies the signed license data and extracts the fields of the response, then
passes them to your Policy for further evaluation.

If the license is valid, the Policy caches the response in
SharedPreferences and notifies the validator, which then calls the
allow() method on the LicenseCheckerCallback object.

If the license not valid, the Policy notifies the validator, which calls
the dontAllow() method on LicenseCheckerCallback.

In case of a recoverable local or server error, such as when the network is
not available to send the request, LicenseChecker passes a RETRY response to
your Policy object's processServerResponse() method.

Also, both the allow() and dontAllow() callback methods receive a
reason argument. The allow() method's reason is usually Policy.LICENSED or Policy.RETRY and the dontAllow() reason is usually Policy.NOT_LICENSED or Policy.RETRY. These response values are useful so you can show
an appropriate response for the user, such as by providing a "Retry" button when dontAllow() responds with Policy.RETRY, which might have been because the service was
unavailable.

In case of a application error, such as when the application attempts to
check the license of an invalid package name, LicenseChecker passes an error
response to the LicenseCheckerCallback's applicationError()
method.

Note that, in addition to initiating the license check and handling the
result, which are described in the sections below, your application also needs
to provide a Policy implementation and, if the Policy
stores response data (such as ServerManagedPolicy), an Obfuscator implementation.

Add imports

First, open the class file of the application's main Activity and import
LicenseChecker and LicenseCheckerCallback from the LVL package.

Java

If you are using the default Policy implementation provided with the LVL,
ServerManagedPolicy, import it also, together with the AESObfuscator. If you are
using a custom Policy or Obfuscator, import those instead.

Java

Implement LicenseCheckerCallback as a private inner class

LicenseCheckerCallback is an interface provided by the LVL for handling
result of a license check. To support licensing using the LVL, you must
implement LicenseCheckerCallback and
its methods to allow or disallow access to the application.

The result of a license check is always a call to one of the
LicenseCheckerCallback methods, made based on the validation of the response
payload, the server response code itself, and any additional processing provided
by your Policy. Your application can implement the methods in any way needed. In
general, it's best to keep the methods simple, limiting them to managing UI
state and application access. If you want to add further processing of license
responses, such as by contacting a backend server or applying custom constraints,
you should consider incorporating that code into your Policy, rather than
putting it in the LicenseCheckerCallback methods.

In most cases, you should declare your implementation of
LicenseCheckerCallback as a private class inside your application's main
Activity class.

Implement the allow() and dontAllow() methods as
needed. To start with, you can use simple result-handling behaviors in the
methods, such as displaying the license result in a dialog. This helps you get
your application running sooner and can assist with debugging. Later, after you
have determined the exact behaviors you want, you can add more complex handling.

Some suggestions for handling unlicensed responses in
dontAllow() include:

Display a "Try again" dialog to the user, including a button to initiate a
new license check if the reason supplied is Policy.RETRY.

Display a "Purchase this application" dialog, including a button that
deep-links the user to the application's details page on Google Play, from which the
use can purchase the application. For more information on how to set up such
links, see Linking to Your Products.

Display a Toast notification that indicates that the features of the
application are limited because it is not licensed.

The example below shows how the LVL sample application implements
LicenseCheckerCallback, with methods that display the license check result in a
dialog.

Kotlin

private inner class MyLicenseCheckerCallback : LicenseCheckerCallback {
override fun allow(reason: Int) {
if (isFinishing) {
// Don't update UI if Activity is finishing.
return
}
// Should allow user access.
displayResult(getString(R.string.allow))
}
override fun dontAllow(reason: Int) {
if (isFinishing) {
// Don't update UI if Activity is finishing.
return
}
displayResult(getString(R.string.dont_allow))
if (reason == Policy.RETRY) {
// If the reason received from the policy is RETRY, it was probably
// due to a loss of connection with the service, so we should give the
// user a chance to retry. So show a dialog to retry.
showDialog(DIALOG_RETRY)
} else {
// Otherwise, the user is not licensed to use this app.
// Your response should always inform the user that the application
// is not licensed, but your behavior at that point can vary. You might
// provide the user a limited access version of your app or you can
// take them to Google Play to purchase the app.
showDialog(DIALOG_GOTOMARKET)
}
}
}

Java

private class MyLicenseCheckerCallback implements LicenseCheckerCallback {
public void allow(int reason) {
if (isFinishing()) {
// Don't update UI if Activity is finishing.
return;
}
// Should allow user access.
displayResult(getString(R.string.allow));
}
public void dontAllow(int reason) {
if (isFinishing()) {
// Don't update UI if Activity is finishing.
return;
}
displayResult(getString(R.string.dont_allow));
if (reason == Policy.RETRY) {
// If the reason received from the policy is RETRY, it was probably
// due to a loss of connection with the service, so we should give the
// user a chance to retry. So show a dialog to retry.
showDialog(DIALOG_RETRY);
} else {
// Otherwise, the user is not licensed to use this app.
// Your response should always inform the user that the application
// is not licensed, but your behavior at that point can vary. You might
// provide the user a limited access version of your app or you can
// take them to Google Play to purchase the app.
showDialog(DIALOG_GOTOMARKET);
}
}
}

Additionally, you should implement the applicationError()
method, which the LVL calls to let your application handle errors that are not
retryable. For a list of such errors, see Server
Response Codes in the Licensing Reference. You can implement
the method in any way needed. In most cases, the
method should log the error code and call dontAllow().

Create a Handler for posting from LicenseCheckerCallback
to the UI thread

During a license check, the LVL passes the request to the Google Play
application, which handles communication with the licensing server. The LVL
passes the request over asynchronous IPC (using Binder) so
the actual processing and network communication do not take place on a thread
managed by your application. Similarly, when the Google Play application
receives the result, it invokes a callback method over IPC, which in turn
executes in an IPC thread pool in your application's process.

The LicenseChecker class manages your application's IPC communication with
the Google Play application, including the call that sends the request and
the callback that receives the response. LicenseChecker also tracks open license
requests and manages their timeouts.

So that it can handle timeouts properly and also process incoming responses
without affecting your application's UI thread, LicenseChecker spawns a
background thread at instantiation. In the thread it does all processing of
license check results, whether the result is a response received from the server
or a timeout error. At the conclusion of processing, the LVL calls your
LicenseCheckerCallback methods from the background thread.

To your application, this means that:

Your LicenseCheckerCallback methods will be invoked, in many cases, from a
background thread.

Those methods won't be able to update state or invoke any processing in the
UI thread, unless you create a Handler in the UI thread and have your callback
methods post to the Handler.

If you want your LicenseCheckerCallback methods to update the UI thread,
instantiate a Handler in the main Activity's
onCreate() method,
as shown below. In this example, the LVL sample application's
LicenseCheckerCallback methods (see above) call displayResult() to
update the UI thread through the Handler's
post() method.

Java

Then, in your LicenseCheckerCallback methods, you can use Handler methods to
post Runnable or Message objects to the Handler. Here's how the sample
application included in the LVL posts a Runnable to a Handler in the UI thread
to display the license status.

Instantiate LicenseChecker and LicenseCheckerCallback

In the main Activity's
onCreate() method,
create private instances of LicenseCheckerCallback and LicenseChecker. You must
instantiate LicenseCheckerCallback first, because you need to pass a reference
to that instance when you call the constructor for LicenseChecker.

When you instantiate LicenseChecker, you need to pass in these parameters:

If you are using ServerManagedPolicy, you won't need to access the class
directly, so you can instantiate it in the LicenseChecker constructor,
as shown in the example below. Note that you need to pass a reference to a new
Obfuscator instance when you construct ServerManagedPolicy.

The example below shows the instantiation of LicenseChecker and
LicenseCheckerCallback from the onCreate() method of an Activity
class.

Note that LicenseChecker calls the LicenseCheckerCallback methods from the UI
thread only if there is valid license response cached locally. If the
license check is sent to the server, the callbacks always originate from the
background thread, even for network errors.

Call checkAccess() to initiate the license check

In your main Activity, add a call to the checkAccess() method of the
LicenseChecker instance. In the call, pass a reference to your
LicenseCheckerCallback instance as a parameter. If you need to handle any
special UI effects or state management before the call, you might find it useful
to call checkAccess() from a wrapper method. For example, the LVL
sample application calls checkAccess() from a
doCheck() wrapper method:

Embed your public key for licensing

For each application, the Google Play service automatically
generates a 2048-bit RSA public/private key pair that is used for
licensing and in-app billing. The key pair is uniquely associated with the
application. Although associated with the application, the key pair is
not the same as the key that you use to sign your applications (or derived from it).

The Google Play Console exposes the public key for licensing to any
developer signed in to the Play Console, but it keeps the private key
hidden from all users in a secure location. When an application requests a
license check for an application published in your account, the licensing server
signs the license response using the private key of your application's key pair.
When the LVL receives the response, it uses the public key provided by the
application to verify the signature of the license response.

To add licensing to an application, you must obtain your application's
public key for licensing and copy it into your application. Here's how to find
your application's public key for licensing:

Go to the Google Play Console and sign in.
Make sure that you sign in to the account from which the application you are
licensing is published (or will be published).

In the Services & APIs page, locate the
Licensing & In-App Billing section. Your public key for
licensing is given in the
Your License Key For This Application field.

To add the public key to your application, simply copy/paste the key string
from the field into your application as the value of the String variable
BASE64_PUBLIC_KEY. When you are copying, make sure that you have
selected the entire key string, without omitting any characters.

Finally, to let the LVL clean up before your application
Context changes, add a call to the LicenseChecker's
onDestroy() method from your Activity's
onDestroy() implementation. The call causes the
LicenseChecker to properly close any open IPC connection to the Google Play
application's ILicensingService and removes any local references to the service
and handler.

Failing to call the LicenseChecker's onDestroy() method
can lead to problems over the lifecycle of your application. For example, if the
user changes screen orientation while a license check is active, the application
Context is destroyed. If your application does not
properly close the LicenseChecker's IPC connection, your application will crash
when the response is received. Similarly, if the user exits your application
while a license check is in progress, your application will crash when the
response is received, unless it has properly called the
LicenseChecker's onDestroy() method to disconnect from the service.

Here's an example from the sample application included in the LVL, where
mChecker is the LicenseChecker instance:

Kotlin

Java

If you are extending or modifying LicenseChecker, you might also need to call
the LicenseChecker's finishCheck() method, to clean up any open IPC
connections.

Implementing a DeviceLimiter

In some cases, you might want your Policy to limit the number of actual
devices that are permitted to use a single license. This would prevent a user
from moving a licensed application onto a number of devices and using the
application on those devices under the same account ID. It would also prevent a
user from "sharing" the application by providing the account information
associated with the license to other individuals, who could then sign in to that
account on their devices and access the license to the application.

The LVL supports per-device licensing by providing a
DeviceLimiter interface, which declares a single method,
allowDeviceAccess(). When a LicenseValidator is handling a response
from the licensing server, it calls allowDeviceAccess(), passing a
user ID string extracted from the response.

If you do not want to support device limitation, no work is
required — the LicenseChecker class automatically uses a default
implementation called NullDeviceLimiter. As the name suggests, NullDeviceLimiter
is a "no-op" class whose allowDeviceAccess() method simply returns
a LICENSED response for all users and devices.

Caution: Per-device licensing is not recommended for
most applications because:

It requires that you provide a backend server to manage a users and devices
mapping, and

It could inadvertently result in a user being denied access to an
application that they have legitimately purchased on another device.

Obfuscating Your Code

To ensure the security of your application, particularly for a paid
application that uses licensing and/or custom constraints and protections, it's
very important to obfuscate your application code. Properly obfuscating your
code makes it more difficult for a malicious user to decompile the application's
bytecode, modify it — such as by removing the license check —
and then recompile it.

Several obfuscator programs are available for Android applications, including
ProGuard, which also offers
code-optimization features. The use of ProGuard or a similar program to obfuscate
your code is strongly recommended for all applications that use Google
Play Licensing.

Publishing a Licensed Application

When you are finished testing your license implementation, you are ready to
publish the application on Google Play. Follow the normal steps to prepare, sign, and then publish the application.

Where to Get Support

If you have questions or encounter problems while implementing or deploying
publishing in your applications, please use the support resources listed in the
table below. By directing your queries to the correct forum, you can get the
support you need more quickly.

Table 2. Developer support resources
for Google Play Licensing Service.